This invention relates to a solution casting process capable of decreasing uneven coating of a functional layer upon applying the functional layer onto a film surface.
Films used in optical field are produced using cellulose triacetate film or the like as the support film, and the cellulose triacetate film and a the like are produced through the solution casting process. In the solution casting process, a polymer solution dissolved in organic solvent is cast from a die onto a support, such as a drum or an endless band, while a ribbon (a film of the solution between the die opening and the landing point of the film at the support) is allowed to adhere closely to the support by a vacuum chamber located in the vicinity of the die. Various improvements have been made in the solution casting process, and for example, Japanese Patent KOKOKU 49-36946 discloses a casting process of a liquid composition so as to contact the flow of the liquid composition closely with a roller by providing a vacuum chamber having two suction rooms, in order to prevent air entrapping. Japanese Patent KOKOKU 62-38133 and 63-57222 disclose a uniform pressing apparatus for a web capable of stabilizing end beads by providing two vacuum zones separated by a partition wall. Japanese Patent KOKAI 5-66212 discloses a casting process which prevents the generation of skimming by using a dope containing a good solvent and a poor solvent at a specific ratio.
However, in the above conventional processes, the ribbon vibrates at a certain frequency caused by the wind pressure vibration, mechanical vibration or the like due to the air column vibration in a vacuum chamber, the air column vibration in a suction duct or the vibration of the vacuum chamber by the vibration of a blower, and thereby, continuous or periodic step-shaped thickness variation occurs.
Incidentally, films used in optical field have an antiglare coating in order to add a hard coating or reflection-reducing to the film. Uneven thickness existing on a support film induces uneven coating of the antiglare layer to degrade appearance and functions of the films. Nevertheless, it is very difficult to remove the above vibration sources.
An object of the invention is to provide a process for solution coating capable of producing a film having a small unevenness in thickness, and thereby, not inducing uneven coating of various functional layers on the film.
The inventors investigated eagerly in order to achieve the above object, and found that the length-of the ribbon influences the generation of the uneven thickness, and the unevenness in thickness can be reduced to the degree of not appreciating uneven coating by adjusting the length of ribbon to a particular range.
That is, concerning step-shaped uneven thickness with a pitch in the range of 5 to 30 mm in the longitudinal direction in the films produced by the solution casting process, the inventors investigated the relationship between the pitch a mm of uneven thickness and (b/c)xc3x97100 (uneven thickness rate) wherein b xcexcm (uneven thickness) is the difference between the highest position and the lowest position in the uneven thickness, and c xcexcm is the (mean) thickness of the support film. The results obtained are shown in FIG. 1. In FIG. 1, ◯ indicates that uneven coating cannot be seen, xcex94 indicates that =uneven coating can be seen slightly, and xc3x97 indicates that uneven coating can be seen, evaluated by visual observation. From the results, they found that, in the range of a pitch of uneven thickness of 5 to 30 mm, uneven coating cannot be seen by rendering the length of a ribbon 3 to 40 mm to complete the invention.
Thus, the present invention provides a process for solution casting which comprises casting a solution of polymer dissolved in organic solvent into a film by extruding from a die onto a support, wherein the length of the film from the die opening to the landing of the film on the support is controlled to 3 to 40 mm.
In the above results of FIG. 1, the vibration sources around the ribbon are necessary to restrict the vibration quantity of the die to 0.02% or less of the distance between the die and the support, the irregularity of the velocity of the support to 0.02% or less of the velocity of the support, and the pressure pulsation in the delivery of the dope to 0.3% or less of the irregularity of the frequency which makes the pitch of uneven thickness 30 mm or more.